Porous structures made by bonding a matrix of particles with bonding material so as to leave the interstices between the particles partially unfilled have been previously taught and used for various purposes, including porous vacuum molds (see Farrell, U.S. Pat. No. 3,166,615) and filters. However, these prior structures have not been impregnated with lubricant, would not be useful as bearings, and in no way predict the improvement in bearings achieved by the invention.
As to bearings themselves, glass microspheres and other particles have previously been used as wear-improving fillers for polymeric bearing structures; see Li, U.S. Pat. No. 3,224,966; Strub, U.S. Pat. No. 3,067,135; Graver, U.S. Pat. No. 3,575,857; Filippov, U.S. Pat. No. 3,843,527; Bre et al, U.S. Pat. No. 3,257,317; and Mizuno, U.S. Pat. No. 3,647,500 (which describes an "oil-free slider" bearing structure made by sintering a mixture of resin and up to 30 weight-percent glass particles; the structure is porous if very large resin particles are used and may then be impregnated with mineral wax). Bearings of the invention contrast with such prior-art suggestions in that the closely packed particles in bearings of the invention provide the matrix of the bearing structure, rather than being dispersed as a filler in a polymeric matrix; and the particles are bonded together by a bonding material that only partially fills the interstices between the particles, leaving a controlled interconnected porosity suitable for storing and feeding a migratable lubricant to a bearing surface. In short, the prior-art bearings are not porous lubricant-impregnated bearings (i.e., do not have the essential properties of conventional porous metal bearings) and do not suggest the advantages obtained by the bearing structure and processes of the present invention.